In an ion trap mass spectrometer, an ion trap is used to confine ions by the action of a high-frequency electric field to separate ions with a specific mass-to-charge ratio (m/z) and further to dissociate such separated ions. A typical ion trap is in 3D quadrupole configuration composed of one ring electrode the inside surface of which is a hyperboloid of revolution of one sheet and a pair of endcap electrodes the inside surface of which is a hyperboloid of revolution of two sheets that are disposed facing each other across said ring electrode. In addition, an ion trap in linear configuration composed of four rod electrodes disposed parallel to each other is also well-known. In this specification, for convenience, a “3D quadrupole configuration” is used as an example for explanatory purposes.
A conventional general ion trap, namely, an analogue-driven ion trap, applies a sinusoidal high-frequency voltage to a ring electrode, forms a high-frequency electric field for trapping ions in a space surrounded by a ring electrode and endcap electrodes, and then traps ions in said space while keeping such ions vibrating by the action of the high-frequency electric field. On the other hand, a newly developed ion trap applies to a ring electrode a square-wave voltage instead of a sinusoidal high-frequency voltage for trapping ions (see, among others, Patent Literature 1 and 2 and Non-patent Literature 1). The ion trap of this kind is called a ‘digital-driven ion trap’, or simply a ‘digital ion trap (“DIT”) as a square-wave voltage with two different voltage levels, high and low, is generally used.
FIG. 7 is a schematic configuration diagram showing a drive in a conventional DIT FIG. 8 is a timing diagram of this drive. As shown in FIG. 7, the ion trap 3 consists of one electrode 31 and a pair of endcap electrodes 32 and 34, and the ring electrode 31 is connected the positive electrode side high-voltage DC power supply 55 and the negative electrode side high-voltage DC power supply 56 via the positive electrode side switch 53 and the negative electrode side switch 54, respectively. The switches 53 and 54 are high-voltage switching devices such as a power MOSFET. The positive electrode side switch 53 and the negative electrode side switch 54 are turned on and off by the control signals on both positive and negative electrode sides provided from a control unit not shown in the figure (see FIGS. 8 (a) (b)). A short-circuit occurs when both switches 53 and 54 are turned on at the same time. To avoid this, a blank period when both control signals are off is set between the period when control signals on the positive electrode side are on and the period when control signals on the negative electrode side are on. It may be supposed that when such control signals are applied, the square-wave voltage applied to the ring electrode 31 will take, as shown in FIG. 8 (c), different levels of voltage, positive voltage (Vdd), negative voltage (−Vdd) and zero voltage; however, there is actually little change in voltage as the electric charges on the ring electrode are trapped when both switches 53 and 54 are off and thus, the shape of such square-wave voltage will be as shown in FIG. 8 (d).
As is seen from FIG. 8, the square-wave voltage applied to the ring electrode 31 becomes a positive voltage (Vdd) for the period TA from the time when control signals on the positive electrode side become on to the time when control signals on the negative electrode side become on, and becomes a negative voltage (−Vdd) for the period TB from the time when control signals on the negative electrode side become on to the time when control signals on the positive electrode side become on. Therefore, if the periods TA and TB are identical, the duty ratio of the square-wave voltage becomes 50%. However, if the period TA becomes different from the period TB due to a gap in the timing of both control signals, or if there are variations in switching characteristics (response time, etc.) of both switches 53 and 54 even when the periods TA and TB are identical, the duty ratio of the square-wave voltage deviates from 50%.
In the DIT, if the duty ratio of a square-wave voltage applied to the ring electrode 31 deviates from 50%, the following problems will occur.
(1) The change of the duty ratio of the square-wave voltage will alter the shape of the stability region in the diagram of stability region for trapping ions that is drawn based on the stability condition for the solution of the Mathieu equation even where there is no change in frequency of the square-wave voltage or any other related factors (see FIG. 2 of Non-patent Literature 2). Under such condition, ions with a high mass-to-charge ratio are likely to escape from the stability region and thus, such ions cannot be confined in the ion trap. FIG. 9 shows an example mass spectrum measured with a mass spectrometer using the DIT, and (b) shows the case where the duty ratio of the square-wave voltage is 50% and (a) shows the case where the duty ratio deviates from 50%. It is clear that in (a), ions with a mass-to-charge ratio of 600 or more are not detected.
(2) As shown in FIG. 2 of Non-patent Literature 2, the relation between the value q and the value β, important parameters for the ion trap, changes with a change in the shape of the stability region in the stability region diagram. This gives rise to a larger difference between the theoretical mass-to-charge ratio of target ions to be trapped and the mass-to-charge ratio of ions actually trapped in the ion trap, and therefore, the correction of such difference (mass calibration) deviates from the theoretical value.
FIG. 10 shows the obtained correction (variation) values of value q when the duty ratio of a square-wave voltage is deviated by 0.3% from 50%. If the original value q is large, the correction values and the mass-dependence are small, but it can be observed that the smaller the value q gets, the larger the correction values and the mass-dependence become. If this gap becomes larger and, in particular, the mass-dependence becomes larger, it becomes more difficult to accurately trap the target ions with a desired mass-to-charge ratio in the ion trap, which then reduces the selectivity of precursor ions, for example.
The 0.3% deviation of the duty ratio assumed in making FIG. 10 referred to above is only for 1.8 nsec if the period of a square-wave voltage is 600 nsec (the frequency is 1.67 MHz). Such minor time lag can be easily caused by characteristic changes, due to the temperature and other factor, of ICs such as drivers and buffers used for a transmission line to provide control signals to operate the switches 53 and 54 and also the switches themselves. Therefore, the problem about the mass calibration referred to above can happen frequently.